The present invention relates to an optical waveguide device and an optical second harmonic generator device in the form of Cerenkov radiation using the optical waveguide device.
Upon the reception of a light radiation of an angular frequency .omega.(2.pi.f, where f is a frequency), an optical second harmonic generator device (hereinafter referred to as a "SHG" generates a second harmonic light radiation of an angular frequency 2.omega.. The optical SHG broadens the wavelength range to expand the laser application field and optimizes the utilization of laser light in various technical fields. For example, the use of laser light of a short wavelength increases recording density in optical or magneto-optical recording and reproducing using laser light.
A Cerenkov radiation type SHG, which applies a fundamental optical wave to a linear optical waveguide formed on an optically nonlinear single crystal substrate and generates a second harmonic wave, is disclosed, for example, in Applied Physics Letters, 17, 447 (1970). This SHG is fabricated by forming a polycrystalline ZnS optical waveguide on a ZnO nonlinear single crystal substrate and is capable of generating a second harmonic having a wavelength of 0.53 .mu.m by using a Nd:YAG laser emitting laser light having a wavelength of 1.06 .mu.m. However, since the waveguide is formed of a polycrystalline material, the SHG suffers from a large propagation loss. Furthermore, since the ZnO substrate has a small nonlinear d-constant, the SHG efficiency is considerably low.
The SHG disclosed in Japanese Patent Laid-open (Kokai) No. 61-189524 employs a LiNbO.sub.3 (hereinafter referred to as "LN") substrate with a proton exchanged LiNbO.sub.3 optical waveguide. This SHG has a high SHG efficiency .eta. of 1% or higher.
The efficiency .eta. of a Cerenkov radiation SHG is expressed by: EQU .eta..alpha.d.sup.2 .multidot.p.sup.w l
where d is the d constant of the nonlinear optical material, p is the power density of the fundamental wave and l is the interaction length. Accordingly, it is necessary for the enhancement of the efficiency to use a material having a large d constant, to increase the power density of the fundamental wave and to increase the interaction length l. The value of the d constant of a material is dependent on the geometric relation between the crystal orientation and the polarization of the fundamental wave. For LN, the maximum is .vertline.d.sub.33 .vertline.=34.4.times.10.sup.-12 (m/V). With proton-exchanged LN, only the refractive index ne for extraordinary light can be increased. Since the surfaces of an X-plate and a Y-plate are roughened by etching during proton exchange, only a Z-plate (a substrate having major surfaces perpendicular to a z-axis parallel to the c-axis) of nonlinear single crystal LN can be used only for a TM mode, and hence a complicated system is necessary to couple the SHG optically to a semiconductor laser light source. The maximum difference .DELTA.n between the refractive index of the waveguide and the refractive index of the substrate is on the order of 0.14. Accordingly, the light confinement performance, i.e., the SHG efficiency, is limited. Furthermore, proton exchange is effective only for LN, and LiTaO.sub.3 or KTiOPO.sub.4 (hereinafter referred to as "KTP") cannot be used.
Since the Cerenkov radiation angle when a SHG employing a LN substrate operates at its maximum efficiency is a comparatively large angle of about 16.degree., the generated second harmonic wave is reflected by the interface between the backside of the substrate and, for example, air, and a plurality of light radiations are emitted when the interaction length 1 of the substrate, i.e., the size of the substrate along the direction of travel of light, is increased to increase the SHG efficiency .eta.. Thus it is necessary to eliminate the reflected light radiations or to increase the thickness of the substrate. A focusing optics for the generated second harmonic light is complicated.
A SHG disclosed in Applied Physics Letters, 50, 1216(1987) employs a nonlinear KTP single crystal substrate. An optical waveguide is formed on the nonlinear KTP single crystal substrate by an ion exchange process. This SHG utilizes mode dispersion in the optical waveguide.
A KTP single crystal has a comparatively large nonlinear constant d.sup.33 of 13.7.times.10.sup.-12 (m/V), is transparent to light radiation in the blue light wavelength range, and has refractive indices n.sub.x =1.748, n.sub.y =1.755 and n.sub.z =1.840 for a light radiation having a wavelength of 0.84 .mu.m, Which are far smaller than those of LiNbO.sub.3 (LN) and LiTaO.sub.3.
However, the SHG efficiency of the SHG employing an optical waveguide formed by an ion exchange process is practically excessively sensitive to parameters including the wavelength of the fundamental wave and the depth of the optical waveguide. Thus, it is rather difficult to practically utilize the ion exchange process. Accordingly, the practical application of a nonlinear KTP single crystal substrate, which has a small refractive index n and is expected to ensure the satisfactory light confinement effect of an optical waveguide formed thereon, to a SHG has been desired.